Simply go to the Chrome store and install the app from this link.
The next question would be, but how do I see their desktop at the same time? Quite simply by opening a new tab a new chrome window while leaving the existing desktop remote running and then start the remote, but this time you would be the one sharing or connecting remote, whichever you did not do initially. Since the program is so simple, you choose whether you want to share your desktop, in which case you provide the code to the person connecting, or you choose to connect to the remote, in which case they provide you with the code.